Materials having relatively high thermal and electrical conductivities have been found desirable for use in microelectronic packaging. Pure Copper, for instance, has been found particularly suitable both as an efficient conductor of electricity and because of its ability to rapidly conduct and dissipate heat.
For this reason, copper sheeting is often used as part of a microchip heat sink/isolator. Typically, the sheeting is bonded to a ceramic substrate by a process known as direct bonding. This process is notable for its products which have a relatively high thermal dissipation capability, strength, reliability, and small size. Direct bonding is also known for its relatively low cost. An intermediate bonding layer is sometimes used to modify the properties of the sheeting material, e.g., to strengthen the material or control its coefficient of thermal expansion.
Direct bonding requires that the bonding surface of the copper be oxidized so that covalent bonds can be formed with the ceramic during the thermal bonding cycle. Typically, both sides of the copper strip are coated with (or dipped in) a chemical solution that promotes oxidation. This forms a low melting temperature eutectic of copper oxide on each side of the strip which bonds to a ceramic substrate upon heating.
While one oxidized side of the strip is joined to the substrate during direct bonding, the other oxidized side bonds undesirably to the carrier or fixture upon which it rests. Also, the resulting bond has been found of limited integrity, dimensional stability, durability and reliability. Chemical coating has additionally been found costly and produces excessive chemical waste.